Membrane current and contraction in frog atrial fibres

Abstract

SUMMARY1. Membrane current and mechanical activity were recorded from short segments of frog atrial muscle strips using a double sucrose gap voltage clamp arrangement. Experiments were performed at 4-7O C. Two types of contraction were observed dependent upon the duration of the clamp.2. Short-lasting depolarizations caused a flow of Ca inward current, ICa, and development of a phasic contraction. Time to peak tension approximated 400 msec. Both ICa and contraction, as functions of membrane potential, had a threshold of about -40 mV and were maximal at inside positive potentials in normal Ringer fluid. Peak tension decreased at strong depolarizations. 3. The minimum time of depolarization required for initiation of a phasic contraction was 40-70 msec. The time necessary for full activation of contraction was 200-300 msec and comparable to the period of time covered by the flow of ICa-4. There was no marked change in peak tension upon repetitive depolarization to the same membrane potential.5. Restoration of (phasic) contractility after a preceding contraction was strongly dependent on the level of membrane potential between conditioning and test pulse. Restoration was half complete at potentials around -45 mV.6. Long-lasting depolarizations generated tonic (sustained) contractions superimposed on the phasic (transient) ones. Threshold potential for initiation of tonic contractions was usually positive to the threshold of phasic contractions. The time taken to attain the final level of tension ranged between 0 7 and 3 sec. Plateau tension, as a function of membrane potential, increased with increasing depolarization and reached a flat maximum at about + 50 mV in normal Ringer fluid. 7. At membrane potentials near zero level, plateau tension developed by the tonic mechanism was about twice peak tension due to phasic contraction.

The calcium and frequency dependence of the slow inward current ‘staircase’ in frog atrium.

“…In this tissue, the slow inward current, first described by Rougier, Vassort, Garnier, Gargouil & Coraboeuf (1969) has been shown to be directly involved in producing phasic tension (Vassort & Rougier, 1972;Leoty & Raymond, 1972;Horackova & Vassort, 1976; (Einwichter, Haas & Kern, 1972), in contrast to the more indirect role suggested for the mammalian myocardium (e.g. Bassingthwaighte & Reuter, 1972;Fozzard, 1977).…”

Calcium‐sodium antagonism on the frog's heart: a voltage‐clamp study.

“…Synchronous measurements of membrane current and tension on various cardiac tissues have revealed some of the features of the excitationcontraction coupling process (Beeler & Reuter, 1970a, b;Goto, Kimoto & Kato, 1971;Morad & Orkand, 1971;Ochi & Trautwein, 1971; Einwachter, Haas & Kern, 1972;Vassort & Rougier, 1972). The general picture emerging from these investigations is that there exist two types of contractile response to depolarization: a phasic tension associated with the slow (secondary) inward current; and a tonic tension which relaxes only upon repolarization.…”

EFFECTS OF pH ON THE PROCESSES OF EXCITATION-CONTRACTION COUPLING OF BULLFROG ATRIUM

“…It is well known that in the frog atrium, a tonic tension component, different from the phasic one related to Ica, is developed when a long and strong depolarizations is applied (EINWACHTER et al, 1972;VASSORT and ROUGIER, 1972). GOTO et al(1974) reported that the Na-Ca exchange mechanism across the surface membrane may be responsible for the development of this tonic contraction.…”

“…In the present experiment, therefore, we attempted to re-examine the effects of pH on excitation-contration (E-C) coupling of the bullfrog atrium, in which the coupling processes have been fairly well clarified (EINWACHTER et al, 1972;GOTO et al, 1972GOTO et al, , 1974. …”